The invention relates to a process for controlling an infusion device, in particular for controlling an infusion device of a type comprising a syringe for medical fluids.
Specifically, though not exclusively, the invention can be usefully applied for controlling a correct positioning, on a machine for extracorporeal blood treatment, of a variable-volume container containing the infusion liquid. In a specific use, the infusion device is for injecting an anti-coagulant into an extracorporeal blood circuit at very low-flow delivery rates.
Extracorporeal treatments usually include removal of blood from a patient, treatment of the blood externally of the human body and reintroduction of the blood into circulation.
Extracorporeal blood is made to circulate through a circuit comprising, in general, an arterial line, or blood removal line, which takes the blood from the patient to a blood treatment device (for example a dialyzer filter) and a venous line, or blood return line, which returns the treated blood to the patient.
To reduce the risk of coagulation of the extracorporeal blood, a known method includes infusion of an anticoagulant (for example heparin) into the extracorporeal circuit, generally into the arterial line, through an infusion line, with relatively low infusion flow rates.
An infusion device which is typically used in this method is a syringe pump, wherein a pushing element, on command of a linear actuator, pushes the plunger of a syringe containing the anticoagulant at an advancement rate which is predetermined and relatively slow. For example, in a dialysis treatment, usually the syringe contains the quantity of anticoagulant necessary for several hours of treatment. The pushing element and the actuator are part of the extracorporeal treatment machine (for example the dialysis machine), while the syringe is generally of the single-use type, or in any case is of the disposable type.
One of the problems of infusion devices having variable-volume containers, such as for example pumps or syringes used for administering an anticoagulant into an extracorporeal blood circuit, concerns the correct positioning of the variable-volume container (syringe) containing the anticoagulant liquid.
An incorrect positioning can be due to various causes, such as, for example:                absence of the syringe;        absence of contact between the syringe plunger and the pushing element, where the pushing element is a part of the infusion device predisposed for exerting a push on a mobile part (plunger) of the syringe to cause the infusion of the liquid contained in the syringe cylinder, in contrast with the action of the extracorporeal circuit pressure;        absence of a connection, or a poor and wrong connection of the syringe cylinder to the cylinder support.        
In relation especially to the second above-cited cause, it is important that the pushing element, immediately after having received the command to start infusion of the anticoagulant liquid, is in stable contact with the mobile part (plunger), and so is able to perform the infusion force.
A failure in infusion, caused by absence of the above-described contact, might be prolonged, even if the positioning is only slightly wrong, given the low infusion flow rate and thus the extreme slowness of the pusher advancement. In a situation such as this, for example, a distance of one millimeter between the pusher and the plunger might cause a delay of several hours in the start of the infusion, with a consequent considerable risk of the formation of large clots in the extracorporeal blood.
Control of the correct positioning of the variable-volume container is at present performed by means of a visual inspection on the part of the operator, who checks if the pusher of the infusion device is in contact with the syringe plunger before initiating the administration of anticoagulant.
To do this, the operator positions the syringe in the appropriate seating predisposed on the machine, then advances the pusher gradually (for example by manually activating a command advance button on the front of the machine), up until he or she can see that contact between the pusher and the syringe has been achieved. At this point the infusion device is considered to be ready for dispensing the anticoagulant liquid.
This solution, however, exhibits various limitations and drawbacks, among which the risk of error on the part of the operator and the fact of complicating the stages necessary for readying an extracorporeal circuit.